A stable supply of light alkane feedstock at large volumes enabled by shale gas production incentivizes new chemical process development for key petrochemicals production. Aromatic hydrocarbons such as benzene, toluene, and xylenes are derived from crude oil through catalytic reforming of straight-run heavy naphtha and steam cracking of light naphtha. Driven by geopolitical instabilities in crude oil production regions, production and supply of crude oil has been unpredictable, and its volatile market price has reached unaffordable levels in short period of times. Therefore, a need for cost-advantaged feedstock with a stable supply has been growing for production of aromatic hydrocarbons.
Aromatic hydrocarbon production from light alkanes offers incentives including an abundant and stable supply of feedstock at competitive prices. Light alkanes can undergo catalytic reactions including dehydrogenation, oligomerization, and cyclization in a complex manner until aromatic hydrocarbons are produced. Measurable quantities of hydrogen, methane, light alkanes other than feed alkane, and light alkenes (or light olefins) are also produced as byproducts. Even though light alkanes offer economic incentives as a new feedstock, there remain technical obstacles for industrial scale production of aromatic hydrocarbons from light alkanes.
Light alkane conversion to aromatic hydrocarbons is a strongly endothermic reaction and, therefore, the process for producing aromatic hydrocarbons from light alkanes requires supplying a large quantity of reaction heat. Approximately 4,399 kJ of thermal energy is required for the reaction heat per kg of benzene produced from ethane. The dehydrogenation step of the light alkane feed is mainly responsible for the reaction heat requirement. Considering the strong endothermic requirement, there needs to be a reliable and efficient method and apparatus for providing reaction heat required for producing aromatic hydrocarbons from light alkanes at an industrially attractive production rate.
The present invention found that another important factor in aromatic hydrocarbon production is uniform catalyst bed temperature in a specific temperature range. Findings of the present invention suggest that light alkane conversion for aromatic hydrocarbons production is highly sensitive to reaction temperature in terms of light alkane conversion rate and catalyst deactivation. If the catalyst bed temperature is below 500° C., light alkane conversion rate is too low to meet commercially attractive production rates. On the other hand, unacceptably fast catalyst deactivation is driven at catalyst bed temperatures higher than 660° C., and renders a catalyst cycle time that is too short between catalyst regenerations for commercial operation. Achieving a uniform catalyst bed temperature in a desired temperature range, preferably between 500° C. and 660° C., more preferably between 520° C. and 640° C., and most preferably between 540° C. and 620° C., in industrial scale reactors is critical for commercial viability of aromatic hydrocarbons production from light alkanes.
Methods for supplying reaction heat have been developed by chemical industry for reactions of an endothermic nature. However, adoption of these methods for aromatic hydrocarbon production from light alkane feedstock yields undesirable operational issues and non-uniform temperature distribution in the catalyst bed. For instance, preheating light alkane feedstock to provide sufficient sensible heat for the endothermic reaction is not feasible because the reaction heat required for industrially attractive rates is substantially larger than the quantity of sensible heat achievable through feedstock preheating. Excessive preheating of the feedstock in an attempt to increase sensible heat and provide the reaction heat required often leads to technical issues, including thermal breakdown of feedstock, accelerated catalyst deactivation, and shortened lifetime of preheating tubes. Heating an inter-stage stream for the next stage reactor in a serially connected multi-stage reactors configuration is not practical either because heating of the inter-stage stream leads to thermal breakdown of the desired product at elevated temperatures and resultant building-up of coke inside the tube.
Intensive heating-up of reactor tubes with a fixed catalyst bed would not be applicable. Catalyst with a fixed position in a stationary state inside an externally heated reactor impedes heat supply itself, and creates non-uniform temperature distribution within the catalyst bed. This leads to accelerated catalyst coking, catalyst sintering problems near the reactor wall, not enough thermal energy to drive the endothermic reaction in the center of the catalyst bed.
Catalyst heating by burning coke while regenerating catalyst (and burning extra fuel when needed) and recycling heated catalyst for reaction heat supply has been explored. Even though circulation of heated catalyst particles from the catalyst regenerator for reaction heat supply has been commercially employed in fluid catalytic cracking (FCC) for heavy portions of crude oil, the same approach would not work properly with light alkanes as feedstock. Light alkane conversion for production of aromatics requires substantially larger amounts of reaction heat than cracking of heavy portions of crude oil when compared on a per unit feedstock mass basis. The present invention also found that light alkane feed produces coke at substantially lower yields than FCC for heavy portions of crude oil. The much stronger endothermic requirement of light alkane feed combined with the substantially lower coke yield makes it impractical to use coke as source of reaction heat supply.
Catalyst deactivation driven by coke formation is another technical hurdle in aromatic hydrocarbon production from light alkanes. Formation of coke over or within the catalyst structure progresses over the course of aromatic hydrocarbon production, leading to a gradual drop in aromatic hydrocarbon production rates. Regeneration of deactivated catalysts makes it difficult or impossible to produce aromatic hydrocarbons from a reactor in a continuous manner and to operate downstream separation units without interruption.
Taken together, there is a need for a new process and apparatus for producing aromatic hydrocarbons from a feedstock of light alkanes by developing a reliable and efficient reaction heat supply to the reactor with uniform catalyst bed temperature in a desired temperature range and by making the entire process continuous.